It is well known that radiation from beta, gamma and other radioactive sources is able to generate light when it strikes certain types of luminescent materials, such as phosphors. The most commonly-used of these radioactive sources is tritium, a weak beta particle emitter.
Conventional luminescent light sources use tritium gas inside a phosphor-coated glass envelope. Typical prior art applications of such light sources are in luminescent safety signs (see, e.g., U.S. Pat. No. 3,409,770), light standards (see, e.g., U.S. Pat. No. 3,889,124), dials and gauges requiring low level high reliability lighting.
A limitation to the extensive use of this technology is that high levels of light intensity are difficult to achieve, owing to the low level of phosphor emissions. Source brightness has remained at relatively low levels, in the range of about 100 to about 800 microlamberts.
In the prior art, concentration of the light has been attempted using reflectors mounted behind the glass tubes. However, this procedure provides no increase in the overall light intensity.
A further problem with the prior art structures is the vulnerability of the enclosure to fracture or breakage and the potential for release of radioactive material. Higher intensity light sources using the conventional structure would require higher levels of radioactivity, thereby increasing the radiation hazard upon fracture or breakage of the enclosure.
In view of these problems with gaseous tritium, attempts have been made to provide the tritium in a safer form by chemical immobilization. One specific prior art disclosure of such attempt in U.S. Pat. No. 2,749,251 discloses a self-luminescent material consisting of an inorganic phosphor intimately mixed with a tritium-containing compound, including a variety of organic compounds, such as alcohols and paraffinic acids. This intimate mixture is coated as a thin layer on the exterior of a light-reflecting surface. This reference does not describe any structure in which a luminescent concentrator is provided nor wherein the tritium is incorporated in a matrix.
Another prior art disclosure is U.S. Pat. No. 3,238,139 which describes a luminescent body consisting essentially of a solid mass of translucent tritium-containing synthetic resin having distributed therethrough particles of a luminous material. This reference, however, does not describe the provision of a luminescent concentrator, so the light produced is quite diffuse.